New U.S. Gulf oil spill report called "ludicrous."

Part of an ongoing series on the environmental impacts of the Gulf oil spill.

As BP finishes pumping cement into the damaged Deepwater Horizon wellhead Thursday, some scientists are taking issue with a new U.S. government report that says the "vast majority" of the Gulf of Mexico oil spill has been taken care of by nature and "robust" cleanup efforts.

In addition, experts warn, much of the toxic oil from the worst spill in U.S. history may be trapped under Gulf beaches—where it could linger for years—or still migrating into the ocean depths, where it's a "3-D catastrophe," one scientist said.

The U.S. government estimated Monday that the Deepwater Horizon spill had yielded about 4.9 million barrels' worth of crude.

Another 25 percent has evaporated into the atmosphere or dissolved in the ocean, and 16 percent has been dispersed via natural breakup of the oil into microscopic droplets, the study says. (Read more about how nature is fighting the oil spill.)

The remaining 26 percent, the report says, is still either on or just below the surface, has washed ashore or been collected from shores, or is buried along the coasts.

Oil Spill Report "Almost Comical"?

For all their specificity, such figures are "notorious" for being uncertain, said Robert Carney, a biological oceanographer at Louisiana State University (LSU) in Baton Rouge.

That's in part because the fluid nature of the ocean means that it's "exceedingly hard" to track oil.

"Water is always moving—if I go out to the spill site tomorrow and look for hydrocarbons, I might not find much, because the oiled water is already gone."

But to accurately figure out how much oil is left, you need to know how much went into the Gulf to begin with, he said.

"Once you start off with that fundamental measure"—the total amount of oil spilled—"being an educated guess, then things aren't that great."

To University of South Florida chemical oceanographer David Hollander, the NOAA estimates are "ludicrous."

"It's almost comical."

According to Hollander, the government can account for only about 25 percent of the spilled Gulf oil—the portion that's been skimmed, burned off, directly collected, and so on.

The remaining 75 percent is still unaccounted for, he said.

For instance, the report considers all submerged oil to be dispersed and therefore not harmful, Hollander said. But, given the unknown effects of oil and dispersants at great depths, that's not necessarily the case, he added.

"There are enormous blanket assumptions."

Oil Trapped Deep in Gulf Beaches

The new report comes after days of speculation about where the Gulf oil has gone. After the damaged well had been capped July 19, U.S. Coast Guard flyovers didn't spot any big patches of crude on the water.

But oil cleanup is mostly getting rid of what's on the surface, Carney said. There's a common perception that "as long as you keep it off the beach, everything's hunky dory," he added.

Microbes are not an oil-cleanup panacea either, LSU's Carney cautioned.

For instance, oil-eating bacteria can't stomach asphalt, the heaviest part of an oil molecule and the same material used to pave roads, he said.

The leftover asphalt falls to the seafloor, where another kind of microbe may chew on it—making the molecule shorter and thus more toxic, according to Carney.

"The sentimentality that bacteria turn everything into fish food and CO2 is total bull," he said.

What's more, microbes cherry-pick whatever piece of oil is easiest to process—and on their own time, said Christopher Reddy, a marine chemist at the Woods Hole Oceanographic Institution in Massachusetts.

Counting on microbes to quickly clean up an oil spill is "like asking a teenager to do a chore. You tell them to do it on a Friday, to do it when it's most advantageous, and they do it on a Saturday," Reddy said.

"It can be frustrating that you can't constrain the role of microbes and overall natural cleanup."

Deep-Sea Oil Spills are "Unchartered Territory"

Another "open question" remains, FSU's Huettel noted: What is happening to the oil deep in the Gulf?

In the cold, dark ocean, this mixture of oil and chemical dispersants may be suspended and preserved, causing long-term problems for deep-sea animals, Texas Tech University ecotoxicologist Ron Kendall said during August 4 testimony before the U.S. Congress.

"We have very limited information on the environmental fate and transport of the mixture of dispersant and oil, particularly in the deep ocean," Kendall said.

Predicting what will happen to the deep-sea ecosystem is "uncharted territory," said Hollander, who's studying what the oil is doing to deep-sea creatures during a series of research cruises this summer and fall.

"Could be a bottom-up collapse, could be nothing happens," he said. But he suspects a "real large chunk of food chain is being disrupted."

"We're getting into something different than the 2-D petroleum spill" on the Gulf's surface, he added. "All of the sudden you've taken this 2-D disaster and turned it into a 3-D catastrophe."